draft-ietf-dhc-agent-subnet-selection-00.txt   draft-ietf-dhc-agent-subnet-selection-01.txt 
Network Working Group Kim Kinnear Network Working Group Kim Kinnear
INTERNET DRAFT Mark Stapp INTERNET DRAFT Mark Stapp
Richard Johnson Richard Johnson
Jay Kumarasamy Jay Kumarasamy
Cisco Systems Cisco Systems
July 2001 November 2001
Expires January 2002 Expires May 2002
Subnet Selection sub-option Subnet Selection sub-option
for the Relay Agent Information Option for the Relay Agent Information Option
<draft-ietf-dhc-agent-subnet-selection-00.txt> <draft-ietf-dhc-agent-subnet-selection-01.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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the subnet information cannot be used as a way to communicate between the subnet information cannot be used as a way to communicate between
the DHCP server and the relay agent. the DHCP server and the relay agent.
Since the relay agent can modify the client's DHCP DHCPREQUEST in Since the relay agent can modify the client's DHCP DHCPREQUEST in
only two ways: the giaddr and the relay-agent-info option, there is only two ways: the giaddr and the relay-agent-info option, there is
thus a need to extend the relay-agent-info option with a new sub- thus a need to extend the relay-agent-info option with a new sub-
option, the subnet-selection sub-option, to allow separation of the option, the subnet-selection sub-option, to allow separation of the
specification of the subnet from the IP address to use when communi- specification of the subnet from the IP address to use when communi-
cating with the relay agent. cating with the relay agent.
2. 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119]. document are to be interpreted as described in RFC 2119 [RFC 2119].
This document uses the following terms: This document uses the following terms:
o "access concentrator"
An access concentrator is a router or switch at the service
provider's edge of a public access network. This document
assumes that the access concentrator includes the DHCP relay
agent functionality.
o "DHCP client" o "DHCP client"
A DHCP client is an Internet host using DHCP to obtain confi- A DHCP client is an Internet host using DHCP to obtain confi-
guration parameters such as a network address. guration parameters such as a network address.
o "DHCP relay agent" o "DHCP relay agent"
A DHCP relay agent is a third-party agent that transfers BOOTP A DHCP relay agent is a third-party agent that transfers BOOTP
and DHCP messages between clients and servers residing on dif- and DHCP messages between clients and servers residing on dif-
ferent subnets, per [RFC 951] and [RFC 1542]. ferent subnets, per [RFC 951] and [RFC 1542].
skipping to change at page 4, line 4 skipping to change at page 3, line 43
A DHCP client is an Internet host using DHCP to obtain confi- A DHCP client is an Internet host using DHCP to obtain confi-
guration parameters such as a network address. guration parameters such as a network address.
o "DHCP relay agent" o "DHCP relay agent"
A DHCP relay agent is a third-party agent that transfers BOOTP A DHCP relay agent is a third-party agent that transfers BOOTP
and DHCP messages between clients and servers residing on dif- and DHCP messages between clients and servers residing on dif-
ferent subnets, per [RFC 951] and [RFC 1542]. ferent subnets, per [RFC 951] and [RFC 1542].
o "DHCP server" o "DHCP server"
A DHCP server is an Internet host that returns configuration A DHCP server is an Internet host that returns configuration
parameters to DHCP clients. parameters to DHCP clients.
o "downstream"
Downstream is the direction from the access concentrator towards
the subscriber.
o "upstream"
Upstream is the direction from the subscriber towards the access
concentrator.
3. Subnet selection sub-option definition 3. Subnet selection sub-option definition
The subnet-selection sub-option MAY be used by any DHCP relay agent The subnet-selection sub-option MAY be used by any DHCP relay agent
which desires to specify a subnet for a DHCP client request that it which desires to specify a subnet for a DHCP client request that it
is relaying but needs the subnet specification to be different from is relaying but needs the subnet specification to be different from
the IP address the DHCP server should use when communicating with the the IP address the DHCP server should use when communicating with the
relay agent. relay agent.
The sub-option contains a single IP address that is the address of a The sub-option contains a single IP address that is the address of a
subnet. The value for the subnet address is determined by taking any subnet. The value for the subnet address is determined by taking any
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a subnet selection option [RFC 3011] as well as a subnet selection a subnet selection option [RFC 3011] as well as a subnet selection
sub-option, the information contained in the subnet selection sub- sub-option, the information contained in the subnet selection sub-
option MUST be used to control the allocation of an IP address in option MUST be used to control the allocation of an IP address in
preference to the information contained in the subnet selection preference to the information contained in the subnet selection
option. option.
When this option is present and the server supports this option, the When this option is present and the server supports this option, the
server MUST NOT offer an address that is not on the requested subnet server MUST NOT offer an address that is not on the requested subnet
or network segment. or network segment.
The IP address to which a DHCP server sends a reply to MUST be the The IP address to which a DHCP server sends a reply MUST be the same
same as it would chose when this option is not present. as it would chose when this option is not present.
4. Security 4. Security
DHCP currently provides no authentication or security mechanisms. DHCP currently provides no authentication or security mechanisms.
Potential exposures to attack are discussed is section 7 of the pro- Potential exposures to attack are discussed is section 7 of the pro-
tocol specification [RFC 2131]. The subnet selection option allows a tocol specification [RFC 2131]. The subnet selection sub-option
relay agent to specify the subnet on which to allocate an address for allows a relay agent to specify the subnet on which to allocate an
a DHCP client. This would allow a malicious DHCP client to perform a address for a DHCP client. Given that the subnet selection option
more complete address-pool exhaustion attack since the client would already exists [RFC 3011], no new security issues are raised by the
no longer be restricted to attacking address-pools on just its local existance of the subnet selection sub-option specified in this docu-
subnet. Under the current DHCP security model there are no methods ment. The existance of either the subnet selection option or subnet
available to circumvent this type of attack. selection sub-option documented here would allow a malicious DHCP
client to perform a more complete address-pool exhaustion attack than
could be performed without the use of these options, since the client
would no longer be restricted to attacking address-pools on just its
local subnet. Under the current DHCP security model there are no
methods available to circumvent this type of attack.
5. Acknowledgments 5. IANA Considerations
Eric Rosen contributed to helping the authors to understand the need IANA has assigned the value of TBD for the relay-agent-info option
for this sub-option. sub-option code for this sub-option.
6. References 6. Acknowledgments
[RFC 826] Plummer, D., "Ethernet Address Resolution Protocol: Or con- Eric Rosen contributed to helping the authors to understand the need
verting network protocol addresses to 48.bit Ethernet address for for this sub-option. Much of the text of this document was borrowed
transmission on Ethernet hardware", RFC 826, November 1982. with only minimal modifications from the document describing the sub-
net selection option [RFC 3011].
7. References
[RFC 951] Croft, B., Gilmore, J., "Bootstrap Protocol (BOOTP)", RFC [RFC 951] Croft, B., Gilmore, J., "Bootstrap Protocol (BOOTP)", RFC
951, September 1985. 951, September 1985.
[RFC 1542] Wimer, W., "Clarifications and Extensions for the [RFC 1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, October 1993. Bootstrap Protocol", RFC 1542, October 1993.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997. Requirement Levels", RFC 2119, March 1997.
skipping to change at page 6, line 26 skipping to change at page 6, line 17
[RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor [RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor
Extensions", Internet RFC 2132, March 1997. Extensions", Internet RFC 2132, March 1997.
[RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP", [RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP",
Internet RFC 3011, November 2000. Internet RFC 3011, November 2000.
[RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC [RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001. 3046, January 2001.
7. Author's information 8. Author's information
Kim Kinnear Kim Kinnear
Mark Stapp Mark Stapp
Cisco Systems Cisco Systems
250 Apollo Drive 250 Apollo Drive
Chelmsford, MA 01824 Chelmsford, MA 01824
Phone: (978) 244-8000 Phone: (978) 244-8000
EMail: kkinnear@cisco.com EMail: kkinnear@cisco.com
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Richard Johnson Richard Johnson
Cisco Systems Cisco Systems
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
Phone: (408) 526-4000 Phone: (408) 526-4000
EMail: jayk@cisco.com EMail: jayk@cisco.com
raj@cisco.com raj@cisco.com
8. Full Copyright Statement 9. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved. Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to oth- This document and translations of it may be copied and furnished to oth-
ers, and derivative works that comment on or otherwise explain it or ers, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and dis- assist in its implementation may be prepared, copied, published and
tributed, in whole or in part, without restriction of any kind, provided
that the above copyright notice and this paragraph are included on all distributed, in whole or in part, without restriction of any kind, pro-
such copies and derivative works. However, this document itself may not vided that the above copyright notice and this paragraph are included on
be modified in any way, such as by removing the copyright notice or all such copies and derivative works. However, this document itself may
not be modified in any way, such as by removing the copyright notice or
references to the Internet Society or other Internet organizations, references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet Stan- which case the procedures for copyrights defined in the Internet Stan-
dards process must be followed, or as required to translate it into dards process must be followed, or as required to translate it into
languages other than English. languages other than English.
The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an "AS This document and the information contained herein is provided on an "AS
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